Measuring the rotation of thousands of stars

Rotation of stars, depending on their surface temperature and gravity (corresponding to their mass and size). Chart: M.B. Nielsen

by Martin Bo Nielsen

Astronomers have been looking at the Sun for thousands of years, trying to understand how it works and what it is made of. Using special equipment these early astronomers noticed small dark spots that moved across the surface of the Sun. Today we know much more about the
Sun and it turns out that these small dark spots are caused by the Suns magnetic field.

As the Sun rotates these dark spots move in and out of view as seen by us here on Earth. The fact that these Sun spots follow the rotation of the Sun tells us that the magnetic field and the
solar rotation are somehow linked. However, fully understanding this relationship is not easy, so we have to look at other stars to see if they also show similar spots and rotate like the Sun.

From 2009 to 2013 the Kepler satellite continuously monitored
more than 150.000 stars, to search for signs of planets like the Earth. This has been a major success in itself, but the data have also been of great interest in other fields of astrophysics,
especially the study of how stars rotate. Just like the Sun, other stars also have small dark spots that follow the rotation of the star. When a star rotates and a spot comes into view the light
of the star becomes a tiny bit dimmer, and when the spot move out of view the stars returns to its normal brightness. The high sensitivity of the measurements from the Kepler satellite allow us
to measure these tiny brightness dips and we can then tell how fast a star rotates. However, just measuring one spot moving across the star is not enough. To be absolutely sure that what we
are measuring is in fact the rotation of the star, we have to be see several spots over many rotations. Luckily the data from the Kepler satellite spans almost 4 years, allowing us to see stars
that have hundreds of spots coming and going over the years. Using this method we have now managed to measure the rotation of tens of thousands of stars, something unheard of just a decade ago,
allowing us to put the Sun in a truly galactic context.